It is known to work bodies of glass or other brittle non-metallic material by removing material therefrom by abrasion or scribing, using diamond or tungsten carbide tools. Such processes involve the expenditure of much time and skill, because they are basically manual.
GB-A-1 254 120 and DE-B-1 244 346 discloses a method of splitting bodies of glass or like material into two parts by a thermal shock process produced by intense local heating of the body by means of an incident beam of coherent radiation, and abstraction of heat from the heat-affected zone in order to produce thermal shock, which causes a crack to extend through the thickness of the body.
In this method the surface of a piece of plate glass is heated by a laser beam of radiation at 10.6 .mu.m wavelength. Some of the beam energy is reflected, while most of it is absorbed and released as heat in a thin surface layer, as thick as one wavelength. The compressive stress produced in the heated layer does not result, however, in the splitting of the glass. Further propagation of heat into the body of the glass is by thermal conduction. The splitting of the plate glass occurs as a considerable volume of the glass is heated up, and the thermally-induced stresses exceed its tensile strength. When a crack starts to form, the point of incidence of the laser beam is already displaced from the edge of the glass. Thus the evolution and propagation of the crack lags behind the movement of the laser spot. The rate of thermal splitting of the glass is rather low, and could not be increased by increasing the laser beam power, because as soon as this power exceeds a certain level, the glass becomes overheated, which is manifested by the formation of longitudinal and transverse microcracks along the line of heating.
The rate of thermal splitting is in inversely proportional to the square of the thickness of the glass to be cut. The thermal splitting rate has also been found to be dependent on the dimensions of the initial glass plate or sheet. The greater the size of the initial plate, the lower is the thermal splitting rate, resulting in a failure to split thermally a glass blank of a size exceeding 500 by 300 mm.
Apart from the low splitting speed, thermal splitting by means of a through-going crack would not provide adequately-high cutting accuracy, for the following reason. The thermal crack starts at the edge of a glass plate. By the time the crack starts, the laser beam has already moved away from the edge of the glass. Within this area, from the glass edge to the laser beam spot, a complex distribution of thermal stresses is produced in the body of the glass and along the line of irradiation before actual splitting starts.
The moment the crack develops, it propagates in leaps through the area where the thermal stresses exceed the tensile strength of the glass. This continues until the crack reaches the area directly adjoining the laser beam spot, where high compressive stresses are concentrated in the surface layers. The crack advances to by-pass the stresses. At this point, the tensile stresses at the beginning of the crack and in the bulk of the glass under the heated surface layer combined to stop further propagation of the crack.
As the crack advances, the edges of the material on both sides of the crack are forced apart, leading to mechanical stresses which assist in further propagation of the crack. In order to ensure accurate splitting, it is essential that the crack-producing forces should be symmetrical with regard to the plane of the crack. This can be easily achieved when the crack is to be along a median plane, in which case the cracks deviate only slightly if at all from the line traced out by the laser beam spot. For this reason, as the crack advances towards a boundary of the plate, the crack curves relative to the path of the laser beam, because of the asymmetry of the thermoplastic stresses.
As has already been mentioned, the rate of laser-induced thermal splitting is dependent on the dimensions of the plate being cut. Thus the rate of thermal splitting of a float glass plate, of 500 by 300 mm size and 3 mm thick, would not be higher than 0.5 mm/s whereas the rate of thermal splitting of a 30 by 100 mm plate of the same material is 8 mm/s.
The rate of thermal splitting also varies at different stages, in particular initially on startup; in the intermediate phase once steady state conditions are reached, and as the cut nears the edge of the glass plate. The speed of relative movement of the glass and of the laser beam spot should increase gradually as the crack advances through the glass.
For these reasons, it is virtually impossible to account for, and adjust properly, the rate of thermal splitting of glass or other brittle non-metallic materials by the known method. Thus, high-quality division and accuracy would not be obtained under real life conditions.
As accumulative outcome of the low speed of laser-induced thermal splitting; poor accuracy, and complexity of the control and adjustment of the thermal splitting parameters, the above method of thermal splitting by a laser beam has not found practical applications, and has been recognize as having poor prospects for the future [Ready, D Industrial Applications of Lasers, Moscow, MIR Publishers, 1981, pp 462-463].
A known method of cutting glass tubes includes the steps of making a score or nick along the would-be line of cut, and then heating the line of cut by a laser beam, with each tube being rotated and simultaneously advanced along the beam, followed by cooling the heated line of cut [SU Inventors' Certificate No. 857 025].
Artificially decreasing the glass strength by scoring the line of cut allows the reliability of the crack development to be enhanced, and reduces the amount of energy required for thermal splitting. As a tube is heated, compressive stresses are produced in the surface layers, and tensile stresses-in the deeper layers. As the heated glass tube is sharply cooled down, its surface layers cool quickly and tend to reduce their volume, while the inner layers oppose this tendency, so that the outer part of the glass experiences tensile stresses. As the tensile strength of glass is substantially lower than its compressive strength, the use of this method of cutting glass tubes improves substantially the efficiency of thermal splitting compared with the conventional techniques of thermal splitting without local cooling of the heated zone.
However, this method of cutting glass tubes could not be applied with adequate efficiency to the splitting of brittle non-metallic materials, such as plate or sheet glass. The underlying reason is that as glass tubes are cut over their entire circumference, by their repeated rotation in a laser beam strip, gradual building up of the thermal stresses takes place. The subsequent local cooling of the line of cut results in the thermal stressing producing a crack which, with the tube rotating, extends around the tube.
If this technique were employed for the thermal splitting of sheet or plate glass, it would not yield any appreciable increase of the cutting efficiency and accuracy, because the shortcomings and limitations already discussed apply in this technique also.
An increase of the cutting speed and accuracy is partly attained by employing the technique disclosed in SU-A-1 231 813, which discloses a method of thermal splitting in which the sheet of glass or like material to be cut is mounted on a rotary table mounted in turn on a coordinate table, and in which the progress of the crack caused by the thermal shock is monitored by a light source and detector from which data are derived to control the movement of both tables. This apparatus makes use of sphero-cylindrical focusing optics allowing an elliptical beam to be thrown on to the surface of the material being cut. This allows the heat-affected zone to be narrowed, and the temperature gradient to be increased, thus enhancing both the rate and accuracy of cutting. In this process, a coolant in the form of a jet of water entrained with air is directed at the heated zone to produce tensile stresses along the line of cut.
However, as in the other known methods of thermally splitting glass, the cutting rate attainable with this technique remains relatively low on account of the poor thermal conductivity of glass and other brittle non-metallic materials, such as glass and other ceramics, or quartz.